328 A CENTURY OF SCIENCE 



A particularly interesting case of isomerism was 

 observed by Wohler in 1828, when he found that ammo- 

 nium cyanate changes spontaneously into urea 



(NH 4 CNO > N,H 4 CO). 



This was the first synthesis of an organic compound from 

 inorganic material, and it overthrew the prevailing view 

 that vital forces were essential in the formation of 

 organic substances. A great many natural organic com- 

 pounds have been made artificially since that time, and 

 some of them, such as artificial alizarin, indigo, oil of 

 wintergreen, and vanillin, have more or less fully 

 replaced the natural products. The preparation of a 

 vast number of compounds not known in nature, many of 

 which are of practical importance as medicines, dyes, 

 explosives, etc., has been another great achievement of 

 organic chemistry. 



The development of our present formulas for organic 

 compounds, by means of which in many cases the rela- 

 tive positions of the atoms can be shown with the great- 

 est confidence, has been gradual. Formulas based on the 

 dualistic idea of Berzelius were used for some time, type 

 formulas, with the employment of compound radicals, 

 came later, the substitution of atoms or groups of atoms 

 for others in chemical reactions came to be recognized, 

 but one of the most important steps was the recognition 

 of the quadrivalence of carbon and the general applica- 

 tion of valency to atoms by Kekule about 1858. This led 

 directly to the use of modern structural formulas which 

 have been of the greatest value in the theoretical inter- 

 pretation of organic reactions. It was Kekule also who 

 proposed the hexagonal ring-formula for benzene, C 6 H 6 , 

 which led to exceedingly important theoretical and prac- 

 tical developments. The details of the formulas for 

 many other rings and complex structures have been estab- 

 lished since that time, and there is no doubt that the 

 remarkable achievements in organic chemistry during the 

 past sixty years have been much facilitated by the use of 

 these formulas. 



Many important researches in organic chemistry have 

 been carried out in the United States, and the activity in 

 this direction has greatly increased in recent years. In 



